One of the questions in my textbook was to arrange the following in order of their reactivity for nucleophilic addition reaction.

$\ce{(p-NO2Ph)_2CO}$

$\ce{(CH3)_2CO}$

$\ce{(p-NO2Ph)COH}$

$\ce{CH3COH}$

I know there are two factors in play here. One is the steric factor (4>3>2>1) and annother is the electropositive nature of the carbonyl carbon, which is the site of nucleophilic attack (1>3>4>2). I am unable to decide which of these factors shall overpower the other and what shall be the final order?

1 Answer
1

My instinct is that there are probably too many variables here for a definitive and accurate answer to be given strictly from first principles. When comparing molecules in which there is only one key difference, especially when that difference leads to steric and electronic effects which are essentially additive, it's fairly easy to draw accurate conclusions. This question, however, is a bit thornier.

Comparing acetone, $\ce{(CH3)2CO}$, to acetaldehyde, $\ce{CH3COH}$, is quite straightforward: acetaldehyde should be the more electrophilic of the two. Firstly, the greater steric bulk of acetone's methyl group slightly hinders nucleophilic attack. Secondly, the additional methyl group is electron-donating by hyperconjugation, comparatively reducing acetone's electrophilicity. Here, both steric effects and electronic effects are synergistic.

The other comparisons are not quite as straightforward, as they require (in the absence of empirical data) qualitative judgment calls to be made.

Consider p-nitrobenzaldehyde. Sterically, reactivity should be reduced due to the obvious bulk of the aromatic ring. Electronically, the picture seems to me to be more complex. From the perspective of valence bond theory, the $sp^2$ carbons may be electron-withdrawing by induction, particularly in concert with the electronegative nitro group. On the other hand, the carbonyl group is also highly electronegative and should remove electron density from the ring, primarily via the $\pi$ system, and I would expect that effect to be greater. Finally, from the MO theory perspective, considering the frontier orbitals, conjugation of the carbonyl to the aromatic ring should result in lowering the energy of the $\pi^*$ LUMO, which ought to enhance reactivity. That, however, may be mitigated by any overall increase in electron density around the carbonyl group. By comparison to acetaldehyde, the steric effects are clearly unfavorable, while the electronic effects are slightly murky.

Finally, for the 4,4'-dinitrobenzophenone, the analysis of electronic effects is roughly the same as for p-nitrobenzaldehyde, but here sterics is undoubtedly going to be the overriding factor, and should result in massively diminished reactivity.

Overall, I would predict the reactivity order to be acetaldehyde >> p-nitrobenzaldehyde > acetone >> 4,4'-dinitrobenzophenone. I don't claim this is in any way authoritative, so I hope others weigh in as well. If I can find some relevant literature containing, e.g., relative rates, I'll update this answer with a link.